OTOH, I was running far too little advance and the car was sluggish (had vaccum hooked up to manifold so static advance was on order of 10 degrees retarded) but no run-on.

Moved the vaccum to the carb and advanced the ignition to "factory" 10 degrees BTDC and it runs much nicer but now diesels when hot.

Interestingly, if I let it idle for a few minutes after stopping and before shutting down it greatly diminishes the event.

You may be running slightly lean, too.

I have noted that lots of SU equipped cars tend to diesel a little. A "fix" I have used in the past was to lower the float level a little so it was less likely to draw fuel on shut-down (MG factory fix was a little thingy that made a huge effective false-air input to prevent run-on... That was due to leaning out the mix for emissions.
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Mike!